PREHEAT OVEN TO 375F. Rinse the chicken pieces dry, pat dry and sprinkle with salt and pepper. Melt the butter over low heat in a large covered casserole dish or Dutch oven on top of the stove. Add the chicken pieces and cook on all sides without browning, about 7 minutes. The chicken should not be crowded, so perform this operation in batches, if necessary. Remove pieces to a plate as they are done. When all chicken is done, add the onions, carrots, celery, bay leaves and thyme and cook about 5 minutes. Sprinkle the flour over the vegetables and cook, stirring for 30 seconds. Add the stock, milk and nutmeg. Increase heat to high, cover and bring to a boil. Replace the thighs and drumsticks, cover and place casserole in the oven for 25 minutes. Add the reserved breasts and cook for another 15 minutes. Remove the casserole from the oven, remove the chicken pieces from the casserole and set them aside. Strain the sauce through a fine sieve and discard onions, carrots, celery and herbs. Keep the chicken pieces warm in the turned-off oven while finishing the sauce. Place strained sauce in a medium saucepan over medium heat and cook until stewing liquid is thick enough to cling to a wooden spoon. To serve, arrange the chicken in a serving casserole or on a deep serving platter and ladle the sauce over it. Accompany the fricassee with noodles or rice pilaf. Refrigerate the meat and sauce separately for up to 3 days. Freeze the meat and sauce separately in 2 batches for up to 3 months.